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PURPOSE: In the differential diagnosis of patients with polyuria-polydipsia one must distinguish usually between primary polydipsia (PP) and central diabetes insipidus (CDI). The first situation is a state of volume expansion and the second of volume contraction. We evaluate whether serum uric acid determination could help to differentiate between the two conditions. PATIENTS AND METHODS: We analyzed the score of 13 consecutive patients with CDI, 7 patients with PP, and 7 patients with nephrogenic diabetes insipidus (NDI). Serum uric acid concentration was available during normonatremia without treatment with 1-desamino-8-D-arginine vasopressin (dDAVP), during mild dehydration and during treatment with dDAVP. In 8 of these patients plasma renin activity (PRA), urate, urea and creatinine clearances were also available. These data were also obtained in the patients with NDI. In 1 patient with CDI, we studied the effect on urate clearance of dDAVP, which stimulates exclusively the [V.sub.2] receptors, and of triglycyl-lysine-vasopressin (TGLV), a potent [V.sub.1]-receptor agonist. RESULTS: Normonatremic polydypsic patients with CDI presented an increase in uric acid concentration (7.1 [+ or -] 2.2 mg/dL), whereas in the PP group the value was decreased (3 [+ or -] 0.75 mg/dL; P [is less than]0.001). All the normonatremic PP presented a serum uric acid concentration lower than 5 mg/dL, whereas all the normonatremic CDI patients, exept 1, presented a value higher than 5 mg/dL. In both groups blood urea concentration was decreased as a consequence of high renal clearances. The hyperuricemia of CDI was related to low uric acid clearances. Patients with hypernatremia and NDI presented a lower increase in serum uric acid concentration than those with similar levels of hypernatremia and CDI (NDI: 5.7 [+ or -] 0.8 mg/dL and CDI: 7.9 [+ or -] 2.3 mg/dL; P [is less than]0.05) and the NDI patients presented an urate clearance corrected for creatinine clearance which was significantly higher than in CDI (9% [+ or -] 3% and 4% [+ or -] 1.1%; P [is less than]0.01). When the patients with CDI were treated with dDAVP and normalyzed their PRA (0.9 [+ or -] 0.4 ng/mL/h) we observed still mild hyperuricemia compared to controls (5.5 [+ or -] 1.4 mg/dL and 4.3 [+ or -] 0.9 mg/ dL; P [is less than]0.01) and a low fractional excretion of filtered uric acid (6.5% [+ or -] 1.7% compared to 8.2% [+ or -] 2% in controls; P [is less than] 0.05). Acute administration of dDAVP, stimulating the [V.sub.2] receptors, in one patient with CDI, had no effect on urate clerance, while TGLV, which stimulates the [V.sub.1] receptor, increased urate clearance. CONCLUSION: The presence of an serum uric acid concentration higher than 5 mg/dL in polyuric polydipsic patients is highly suggestive of CDI. Even when these patients are treated with dDAVP many of them remain hyperuricemic, and this seems to be the consequence of a lack of [V.sub.1] receptor stimulation.

ABSTRACTThe differential diagnosis between hypertonic, isotonic and hypotonic hyponatremia are presented. The help of some usual serum (urea, uric acid and TCO2) and urine parameters (mainly osmolality and sodium concentration) are discussed and help to determine the best treatment. Morbidity associated with untreated hyponatremia and with the different treatment available is also discussed. Who to prevent and treat ODS (osmotic demyelating syndrome) is recalled. The pathophysiology and treatment of hypernatremia are also discussed.